1. Field of the Invention
The present invention relates to a solid state imaging apparatus employing a light-receiving element which is internally amplified and in which channel current is controlled by variations in the potential which is caused by the charges accumulated as a result of light illumination. More particularly, the present invention pertains to an improvement in a signal outputting circuit for a solid state imaging apparatus of this type.
2. Description of the Prior Art
Solid state imaging apparatuses which employ as an internal amplification type light-receiving element a certain type of element known as charge modulation device have been proposed in, for example, the specification of Japanese Patent Laid-Open No. 84059/1986 filed by the present applicant.
FIGS. 1A and 1B are respectively plan and perspective views of an example of the structure of such a solid state imaging apparatus which employs charge modulation devices as the picture elements. As can be seen from the drawings, the apparatus includes a p.sup.- substrate 101 made of a silicon, a channel layer 102 which is an n.sup.- epitaxial layer formed on the substrate 101, a source region 103 which is an N.sup.+ diffused layer, a shallow drain region 104 which is an n.sup.+ diffused layer, a deep drain region 105 which is an n.sup.+ diffused layer and which functions as an isolation region, an insulating film 106, a gate electrode 107 formed in such a manner as to surround the source region 103, a common gate line 108, a common source line 109, a wiring 110 which is a metal thin film for connecting each of the gate electrodes 107 and the gate line 108, and a source electrode 111. The source region 103, the gate electrode 107, and the shallow and deep drain regions 104 and 105 are disposed in a concentric manner as viewed from above and in combination form a light-receiving element which constitutes a picture element. The common gate line 108 serves to connect the gate electrodes 107 of the individual picture elements in a horizontal direction. The common source line 109 is adapted to connect the source regions 103 in a vertical direction.
Such a light-receiving element operates as a bulk channel MOS transistor when it receives light and a signal is accordingly read out: the holes generated as a result of light illumination are accumulated immediately below the gate electrode 107 so as to form an inversion layer. While no inversion layer is being formed, a potential barrier is formed in the bulk channel by the negative potential applied to the gate electrode 107, so no electron current flows from the source region 103 to the drain regions 104 and 105. On the other hand, the inversion layer which is formed by the light illumination reduces the height of the potential barrier formed in the bulk channel, causing the electron current which has been modulated in accordance with the number of holes in the inversion layer to flow.
In consequence, if the gate line 108 is connected to a vertical scanning circuit with the source line 109 being connected to a horizontal scanning circuit through a MOS type selection switch, the source current of the picture element in the row selected by the horizontal scanning circuit from among the picture elements connected to the gate line selected by the vertical scanning circuit is made to flow through a video line to a load so as to enable the intensity of light which is incident to be detected as a variation in the voltage.
In the solid state imaging apparatus of the above-described type, since the signal current from each of the light-receiving elements is read out through the corresponding MOS type selection switch opened and closed by the horizontal scanning circuit to the video line which has a relatively large parasitic capacity (about 1 pF to 100 pF), an amplifier which can detect current at a low impedance is required as a signal detection amplifier.
To deal with this situation, therefore, it is known to convert a signal current I.sub.s to a voltage in the manner expressed by equation (1) below by conducting a negative feedback on an operational amplififer 206 by an impedance 207 (Z.sub.f), as shown in FIG. 2. EQU V.sub.0 =-Z.sub.f .multidot.I.sub.s ( 1)
In FIG. 2, a reference numeral 201 denotes a solid state imaging apparatus employing a charge modulation device; 202 denotes a light-receiving element; 203 denotes a MOS type selection switch; 204 denotes a video line; and 205 denotes a video line parasitic capacity.
However, the solid state imaging apparatus which employs as the picture elements the internal amplification type light-receiving elements of the above-described type has the following disadvantages: firstly, the quantity P.sub.IN of light which is illuminated to the internal amplification type light-receiving element and the gate potential variation V.sub.PH caused by the charges generated as the result of light illumination have a proportional relationship as follows: EQU V.sub.PH .varies.P.sub.IN ( 2)
On the other hand, the gate potential variation V.sub.PH and the resultant signal current I have a non-linear relationship, as shown in equation (3). EQU I=f (V.sub.PH) (3)
Therefore, the signal current I must be converted to a signal which is proportional to the quantity of light which is illuminated P.sub.IN in a subsequent signal processing system, so a complicated circuit is required for accurate conversion to take place. Furthermore, in the internal amplification type light-receiving element, variations in the gate potential often cause variations in the signal current which are larger than those obtained when these factors are proportional to each other, thus requiring a subsequent signal processing system which has an extremely wide dynamic range.
This is explained with reference to FIG. 3. In a solid state imaging apparatus which employs internal amplification type light-receiving elements whose gate potential variation V.sub.PH and the resultant signal current I have a relationship such as that expressed by the equation (4) below, EQU I.varies.(V.sub.PH).sup..gamma. ; .gamma.=2.0 (4)
if the light illumination requires a dynamic range of 40 dB, a dynamic range of 80 dB is required for the detection system of the signal current I, making practical use of a solid state imaging apparatus which employs internal amplification type light-receiving elements difficult.
When the signal current I is to be linearly processed in the subsequent circuit, since the non-linearity thereof consists in the operational characteristics of the light-receiving element, as shown in the equation (3), an inverted conversion conducted by a circuit which employs a known element may cause errors. Further, it is very difficult to perform inverted correction of variations in the temperature caused by the non-linearity expressed by the equation (3).